The present invention relates to a pattern observation method for performing a pattern observation using a charged particle beam.
FIG. 1 illustrates a technical concept of a conventional alignment exposure using an electron beam.
As is shown in FIG. 1, a sample to be aligned is formed such that an insulator film 102 and a resist 103 are provided on a silicon substrate 101. An underlying mark 104 is formed in the insulator film 102 on a surface of the silicon substrate 101.
An electron beam 105 is made to scan the underlying mark 104 located at a deep position from the surface of the resist 103. Reflected electrons 106 from the underlying mark 104 is detected by a detector 107. Based on a detection signal, alignment exposure is carried out. However, when the energy of the electron beam 105 is low, the range of electrons is short. Consequently, the electrons cannot reach the underlying mark located at a deep position from the surface of the resist 103.
In order to solve this problem, an alignment exposure method as illustrated in FIGS. 2A to 2D has been proposed. FIG. 2A illustrates a technical concept of the alignment exposure. A sample to be aligned is the same as shown in FIG. 1.
An electron beam 105 having a predetermined acceleration voltage is radiated on an underlying mark 104. Thereby, a charged portion 112 created by the electron beam 105 appears on the surface of the sample. A difference between electrostatic capacitances 113 and 114 occurs between a region where the mark 104 is formed at a deep position from the surface of the sample and a region where it is not formed, due to an unevenness or a nonuniformity in material of a pattern. The difference in electrostatic capacitance causes a surface potential difference in the charged portion 112 on the surface of the sample.
The surface potential difference appears as a voltage contrast image of secondary electrons 115 at the time of the radiation of the electron beam 105. The contrast image is detected by the detector 107. Thus, the position of the underlying mark 104 can be detected and the alignment in the electron beam exposure can be effected.
FIG. 2B shows a surface potential of the sample in a case where the sample is charged with positive electricity. A region where the surface potential is high corresponds to the portion at which the underlying mark 104 is formed. FIG. 2C shows a secondary electron waveform based on the surface potential difference of the sample in a case where the sample is charged with positive electricity. A region where the amount of secondary electrons greatly decreases corresponds to the portion at which the underlying mark 104 is formed.
If the same charging phenomenon is utilized, a misalignment measurement in a lithography step of a semiconductor fabrication process can be carried out using a scanning electron microscope (SEM). FIGS. 3A and 3B illustrate a technical concept of a misalignment measurement utilizing the charging phenomenon. FIG. 3A is a plan view of misalignment measuring marks, and FIG. 3B is a cross-sectional view of the misalignment measuring marks. In FIGS. 3A and 3B, reference numeral 151 denotes a silicon substrate, 152 a silicon nitride film, 153 a silicon oxide film, 154 an anti-reflection film, 156 a first mark formed on the underlying silicon substrate, and 157 a second mark formed of photoresist. The first mark 156 is formed by removing portions of the silicon substrate and silicon nitride film. The silicon oxide film 153 is formed over the entire surface of the substrate such that the first marks 156 are buried. A top surface of the silicon oxide film 153 is flattened by chemical mechanical polishing (CMP). A misalignment inspection is performed by scanning an electron beam across the first mark 156 and second mark 157. A scanning path is indicated by an arrow 158 in FIG. 3A. Thus, a signal waveform of secondary electrons having peaks near the first mark 256 and second mark 157 can be obtained.
The above method, however, has the following problem.
FIG. 2D is a view for explaining the problem with the conventional alignment method utilizing the charging phenomenon. Specifically, FIG. 2D illustrates a relationship between a radiation time and a surface potential of a sample. Assume that the sample is charged with positive electricity. A solid line indicates a surface potential of a region where the underlying mark 104 is not formed, and a broken line indicates a surface potential of a region where the underlying mark 104 is formed. In order to enhance a signal-to-noise ratio (S/N) at a position of the underlying mark 104 for alignment, it is necessary to scan the beam over the underlying mark 104 several times and to average and add the detection signals.
However, the aforementioned phenomenon utilizing the charging is a temporally transient one, as shown in FIG. 2D. When the radiation time is divided into time periods t1 to t3, a sufficient surface potential difference is obtained in radiation time period t2 and a mark image with full contrast can be observed in this radiation time period.
In radiation time period t3, the charge is excessively high. As a result, only a small surface potential difference is obtained, and a mark image becomes difficult to observe. By contrast, in radiation time period t1, the amount of the radiation beam is small and the charging phenomenon itself will hardly occur. In this time period, it is difficult to observe the mark image. On the other hand, if the beam current for observing the mark image is too high, excessive charging occurs in a short time and the length of the time period t2 in which the mark image can be observed is decreased. If the beam current for observing the mark image is too low, the length of the time period t1 in which the mark image cannot be observed is increased and quick observation of the mark image cannot be carried out.
The optimal condition for mark detection varies depending on the thickness and kind of the insulator film formed over the underlying mark 104. However, as is understood from the above-described problem, it is difficult, in fact, to set the condition for image observation. The same problem as with the alignment exposure also arises in the misalignment measurement as illustrated in FIGS. 3A and 3B.
Another problem with the alignment exposure will now be described.
In usual cases, when alignment exposure is performed, an electron beam is scanned in a single direction to detect the mark on the sample. When the electron beam is scanned, all secondary electrons produced by the radiated electron beam do not enter the detector. In addition, where the surface of the sample has been charged with the electron beam radiated immediately before, such a phenomenon occurs that the secondary electrons re-enter, in particular, the surface of the sample. If the detected secondary-electron image is observed, a dark portion appears on a peripheral portion of the pattern. This is due to the re-entrance of secondary electrons.
If this problem is studied in greater detail, it is understood that the secondary electrons re-entering the surface of the sample travel asymmetrically. Specifically, if an electron beam is scanned in a single direction, the electron beam radiated immediately before charges the surface of the sample negatively, on which the electron beam has been radiated immediately before. On the other hand, the surface of the sample, which has not yet been scanned by the electron beam, is less charged. If this phenomenon is left as it is, the detected secondary-electron signal waveform becomes asymmetric, and a read error of the mark position will occur. Of course, this problem applies to the misalignment measurement.
As has been described above, in the conventional alignment method for the electron beam exposure, the mark located at a deep position from the surface of the resist can be made detectable by utilizing the charging phenomenon. However, the charging phenomenon will easily vary with the passing of time, and it is difficult to precisely detect the mark. Similarly, in the misalignment measurement utilizing the charging phenomenon, it is difficult to precisely detect the mark.
In addition, because of the kind of the method of scanning the electron beam in the alignment measurement, the secondary-electron signal waveform becomes asymmetric, and a read error of the mark position will occur.
The object of the present invention is to provide a pattern observation apparatus and a pattern observation method capable of detecting or measuring an alignment mark with higher precision.
According to an aspect of the present invention, there is provided a pattern observation apparatus for observing a pattern by radiating a charged particle beam on a sample in which the pattern is formed on a substrate and a first film is formed on the substrate including the pattern, the apparatus comprising: a first beam radiation section for performing a first charged particle beam radiation on the sample including the pattern, and charging a surface of the sample; a second beam radiation section for scanning the charged particle beam over the pattern under conditions different from conditions for the first charged particle beam radiation; and an observation section for observing the pattern by detecting secondary electrons from the surface of the sample.
In this invention, the beam radiation step for pattern detection is divided into a first beam radiation step and a charged particle beam scanning step. The sample surface is charged with the first beam radiation, and the charged particle beam is scanned to acquire a pattern image on the sample surface. Thereby, high-precision pattern detection can be performed. Specifically, since the first film surface near the pattern formed on the substrate is sufficiently charged by the first beam radiation, an adequate contrast is produced between the portion on the pattern and the other portion. Accordingly, where the pattern image is obtained by the scan of the charged particle beam, high-precision pattern detection can be performed. In particular, detection of the pattern is not performed in the undesirable condition with a low potential contrast due to deficient charging. The pattern detection according to this invention is applicable to both the alignment exposure and misalignment measurement.
The sample is charged in the following method. FIG. 13 shows an example of the relationship between the acceleration voltage of the electron beam and the emission efficiency of secondary electrons. In this case, the sample is a resist, the abscissa indicates the acceleration voltage, and the ordinate indicates the emission efficiency of secondary electrons from the sample surface. The resist is positively charged with the range of the acceleration voltage between 400V and 1000V so that the emission efficiency of secondary electrons emitted from the surface may exceed 1.
On the other hand, if the acceleration voltage is less than 400V or more than 1000V, the emission efficiency of secondary electrons decreases below 1 and the sample is negatively charged. Specifically, in order to positively charge the sample, the acceleration voltage needs to be chosen from the range of about 400V to 1000V. In order to negatively charge the sample, the acceleration voltage needs to be chosen out of this range. The range of acceleration voltage in which the emission efficiency of secondary electrons exceeds 1 varies depending on samples, but the relationship between the acceleration voltage and the secondary electrons is substantially the same.
According to another aspect of the invention, there is provided a pattern observation apparatus comprising: a table generating section for generating a table in which a scan order is associated with scan positions; a charged particle beam scanning mechanism for scanning, according to the table, a charged particle beam over a sample on which a pattern is formed; a detection mechanism for detecting secondary electrons produced from the sample by the scanning of the charged particle beam, and outputting secondary electron detection signals; an image information generating section for rearranging the secondary electron detection signals in association with the scan positions on the basis of the table, thereby generating image information of a surface of the sample; and a pattern position determination section for determining a pattern position on the basis of the image information.
Thus, even if the charged particle beam is made to scan in any scan order, the image of the sample including the pattern can be acquired, and accordingly the pattern can be observed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.